When filling the fuel tank of a motor vehicle, it is common to recover the vapor escaping the tank when filling it with liquid fuel from a fuel pump unit, this since gasoline and diesel vapors contain hydrocarbons that are harmful to inhale and have negative effects on the environment. The vapor recovery is performed by a system integrated in the fuel pump unit. The system usually comprises a pump for drawing vapor from the tank and vapor measuring means which measures the volume of vapor recovered and compares it with the volume of dispensed fuel. Adjusting means are provided which adjust the flow rate of the vapor to match the flow rate of the fuel. An example of such a system is described in European patent application EP1460033. Such a system normally comprises a number of valves and other components, which should function properly in order for the vapor recovery system to work as it is supposed to. Though not described in EP1460033, a prior art vapor recovery system comprises a monitoring system which is set to shut off the fuel pump in case a failure is detected. The monitoring system comprises a central unit connected to the vapor measuring means, and to sensing means for sensing whether the previously mentioned valves and other components are functioning properly. The monitoring means further comprises some kind of electronic means for shutting off the fuel pump. The means for shutting off the fuel pump shuts off the pump when the central unit determines that the measuring means is detecting an alarmingly small volume of recovered vapor, or when the sensing means detect failure in any of the monitored components. Any of these events is in other words interpreted as a signal that the vapor recovery is not working. However, such monitoring systems are also sensitive to low temperatures. This since the valves and other components which are equipped with sensing means, as well as the sensing means and the ingoing parts of the measuring means, are sensitive to cold. Thus, on a number of occasions, fuel pumps have been known to shut off without due cause, which causes great inconvenience for a customer using the pump, especially in an area where the petrol stations are scarce and the climate is harsh with low temperatures.